Recently, a copying machine as an image forming apparatus is used together with a document circulator and a sheet postprocessing apparatus in order to automate a copying operation and sheet postprocessing such as binding or punching the copied sheets. The document circulator, which is provided on a document stand of the copying machine, for example, carries a plurality of documents onto the document stand one by one. Meanwhile, the sheet postprocessing apparatus binds or punches the sheets, which are discharged from the copying machine after the document image is copied, for each bundle of sheets which is consisted of a predetermined number of sheets.
As to this kind of conventional sheet postprocessing apparatus, for example, as disclosed in European Patent Publication No. 346,851, a device, which drives an endless transferring belt in one direction and discharges bound sheets onto a discharge tray from a sheet receiver by means of a pushing member provided on the transferring belt, is known. Furthermore, there has disclosed that the discharge tray is vertically movable and the height of the piled sheets is always kept substantially equal with that of the sheet receiver.
In addition, a device which has been disclosed in European Patent Publication No. 371,403 is known. In this device, a discharge tray is vertically and horizontally movable, and sheets are stored across a sheet receiver and the discharge tray. After one cycle of a copy operation is completed, a bundle of sheets are bound so that the bound bundle of sheets are discharged onto the discharge tray by using a discharge roller and an oscillating roller, or an ejector. Furthermore, there has disclosed that in a normal processing mode, the oscillating roller contacts with the discharge roller so that the sheets are discharged onto the discharge tray from the copy machine main body, whereas in a sheet postprocessing mode, the oscillating roller departs from the discharge roller so that the sheets are temporarily discharged on the sheet receiver from the copy machine main body, and thereafter, the oscillating roller contacts with the discharge roller so that the sheets are discharged onto the discharge tray.
In addition, as disclosed in U.S. Pat. No. 5,137,265, in the sheet postprocessing mode, recesses are provided on the discharge tray for staples to bind the sheets. Moreover, in the sheet postprocessing mode, if a size of sheets to be transported is larger than a predetermined size, the sheets across a sheet receiver and the discharge tray are discharged onto the discharge tray by the discharge roller and an oscillating roller. On the contrary, if the size of sheets to be transported is smaller than the predetermined size, the sheets remain across the sheet receiver and the discharge tray so as not to be discharged onto the discharge tray. Further, in the sheet postprocessing mode, an auxiliary tray is moved, and the discharge tray is placed on an extended line of the sheet receiver so that the sheets which have undergone the sheet postprocessing are discharged from the sheet receiver onto the discharge tray.
In the conventional sheet postprocessing apparatus which has been described above, as shown in FIG. 11(a), copied sheets S, which have been transported from a copy machine main body through a sheet postprocessing transport path 81, are dropped on a sheet receiver 86 by a transport roller 83 so as to be stored. Here, in the case where the sheets S are discharged without undergoing the sheet postprocessing, a driven roller 94 is descended towards a discharge roller 93 by an offset guide 95 so that the sheets S from the sheet postprocessing transport path 81 are caught between a discharge roller 93 and the driven roller 94, and discharged onto the discharge tray 96.
A judgement is made by a sheet detection sensor 91 provided on the sheet receiver 86 as to whether or not the sheets S exist on the sheet receiver 86. If a judgement is made by the sheet detection sensor 91 that the sheets S exist on the sheet receiver 86, a stapler device 89 built in a sheet postprocessing apparatus 80 binds the sheets S to make a book. The sheet detection sensor 91 is installed in the vicinity of the backward end stopper 90 which positions the backward end of the sheets on the sheet receiver 86 so as to be capable of detecting sheets having any size.
As shown in FIG. 11(b), a pusher 92 which contacts with the backward end of the sheet transporting direction advances along the sheet receiver 86 so that the bound sheets S are discharged onto the discharge tray 96. Furthermore, at this time, the discharge roller 93 provided on the forward end of the sheet receiver 86 rotates so as to help the sheets S to be discharged. After the sheets are discharged, as shown in FIG. 11(c), the pusher 92 retreats to its original position and is in a waiting state until the sheets are transported by the next copying operation.
However, the conventional sheet postprocessing apparatus 80 has a problem that a normal and stable sheet postprocessing cannot be carried out.
In other words, for example, in the case where the sheets S having a longer length than the sheet receiver 86 are stored, as shown in FIG. 11(d), since the forward end of the sheets S which is projected from the sheet receiver 86 descends because of its weight, bending is liable to occur on the sheets. As a result, a clearance might be caused between the sheet detection sensor 91 and the sheets S due to the bending of the sheets. If such a clearance is caused, the sheet detection sensor 91 cannot detect the existence of the sheets S. Therefore, even though the sheets S actually exist, there arises a trouble that the sheet postprocessing apparatus such as a stapler device does not operate due to a detection error by the sheet detection sensor 91.
In addition, since storing displacement of the sheets S on the sheet receiver 86 is liable to be caused due to such bending, even though the sheet detection sensor 91 normally operates, there also arises a problem that the sheet postprocessing is carried out with the sheet arrangement incomplete. Further, displacement is liable to occur also during the sheet postprocessing.
Moreover, in the case where a trouble such as jam in the document circulator occurs in the copy machine main body, the normal copy operation cannot be carried out, and invalid sheets which are blank are transported to the sheet postprocessing apparatus 80. However, in the conventional sheet postprocessing apparatus, since such invalid sheets are also stored on the sheet receiver 86 in the like manner of valid sheets where an normal image has been formed, the invalid sheets cannot be separated from the valid sheets, thereby causing troubles that the order of the sheets which have undergone the sheet postprocessing is inaccurate, the invalid sheets are mixed up with the valid sheets. etc.